Method of making composite printing plates



' Aug. 15, 1944. y c55?. BouTwELL 2,355,949

. METHOD OF MAKING COMPOSITE PRINTING PLATSl Filed mi@ 29, 1941 fPatented Augjls, 1944 NUNITED STATES PATENT OFFICE- METHOD OF MAKING(')OMPOSITE PRINTING PLATES Clarence E. Boutwell, Birmingham, Ala.

` Application August 29, 1941, Serial No. 408,828

3, Claims.

Thisinvention relates to printing plates, more particularly to acomposite printing plate embodying a facing made of plastic compositionand bearing the printing surface, and a metallic backing, and has forone of its objects the provision of improved means f or securing thefacing to the metallic backing.

A further object of my invention is -to provide an improved method of Ymaking a composite printing plate embodying a thermoplastic facing and ametal backing, in which the printing surface is molded on the facingbefore it is joineg the backing.

A more particular object of my invention is to provide a compositeprinting plate embodying a plastic facing carrying a printing surfaceand a curved metal backingA adapted to be attached to a rotary printingpress, and in which the plastic facing shall be extremely resistant tothe stresses imposed by the printing operation tending to separate thefacing from the backing.

A still more s pecic object of my invention is to provide a-printingplate embodying a plastic 'facing and a metal backing in' which theplastic facingshall be firmly joined to the metal backing, but may bereadily removed and the backing reused.l

It has heretofore been proposed to provide plastic printing plateshaving metal backings, particularly forV use on rotary printing pressesfo'r the reason that it is difcult to secure a wholly plastic'printingplateto such a press. The usual method proposed for securing the plateto the backing has been by some sort of cement. I have loose under thesevere pounding to which the plate is subjected on the press.

In accordance with my invention, I prepare a metal backing of therequired curvature for attachment to the cylinder of a'rotary printingpress, and then prepare the curved surface in the same manner that aphoto engraver prepares a surface for etching; that is, the curved metalsurface is provided with a sensitized film and is subjected to lightthrough a photo engravers screen, preferably a relatively coarse screensuch as- 40 line, and is etched with acid so as to form a pattern ofstrong undercut shoulders. The reverse undercut dot design of pattern ispreferably employed as affording better adhesion, though a half tonedesign may be used. The plastic printing plate is then prepared in theusual manner,

found that While cement will successfully hold a p rubber printing plateto a metal backing, I have been unable'to nd a cement which will so holda rigid plastic plate to the backing. In the case of rubber plates, theresiliency of the plate provides a cushioning effect which protects the-cement, but "with a rigid plastic printing plate,

for example, asset forth in my prior Patent No. 2,219,587, and-ismachined to the proper thickness. The metal backing is placed in asuitable press and heated to a temperature above the plastic compositionenters the undercut recesses e ofthe backing and is firmly joinedthereto.

I have found that the method just described of joining the printingplate to the backing does not appreclably decrease the thickness of theprinting plate. If it; is desired to maintain the thickness of theprinting plate constant, the undercut recesses of the backing may befirst lled with a plastic compatible with the composition forming theprinting plate, Vbefore the latter is pressed against the backing. Whenthis method of application is employed, it will be seen that there willbe no change in the thickness of the plastic facing. I have found thatprinting plates mounted as described withstand the heavy strains imposedby the action of a rotary printing press for the life of the printingsurface of the plate.

The facing of my improved printing plate may be readily removed byheating the plate to a temperature above the softening point vof theplastic composition forming the plate, yand stripping it off. Otherwise,it can not be removed without lmutilation of the facing and prbabledamage to the backing. By heating and removing the facing, the backingmay be used over and over.

Features of. rry invention are illustrated in the Fig. 1 is a viewillustrating the process of se-' curing the plastic sheet to the metalbacking;

Fig. 2 is a sectional view taken along the line II-II of Fig. 1;

Fig. 3 is a fragmentary sectional view drawn to an enlarged scaleshowing my improved composite printing plate, the plastic facing beingmounted on a metal backing etched with reverse undercut dots;

Fig. 4 is a sectional plan view of the portion of the plate shownin Fig.3, and taken-on line IV--IV of Fig. 3;

Fig. 5'is a sectional view similar'to. Fig. 3, but showing the metalbacking etched with high light half tone dots;

. Fig. 6 is a sectional plan view of the portion of the plate shown inFig. 5 taken -along line VI-VI of Fig. 5;

Fig. -7 is a sectional view taken on line VII-VII of Fig. 8. showing afurther means for securing the plastic facing to the metal backing,especially along the edges of the plate; and

Fig. 8 is a plan viewof the back of the printing plate, drawn to areduced scale, and showing the arrangement of the recesses shown in Fig.7.

Referring to the drawing, I show in Fig. l

a conventional press I0, which may be of any suitable design, and havingan upper platen II and a lower platen I2. On the lower platen I2 rests ahollow cradle I3 having its upper surface curved to the requiredcurvature for the printing plate being made. The cradle I3 is providedwith pipe connections I6 and I1 for circulating water therethrough tomaintain the printing surface of the facing cool while it is beingmounted. The plastic facing bearing the printing surface is shown at I8,and the curved metal backing, prepared as already described, at I 9.Against the metal backing I9 is pressed a hollow shell '2| having itsunder surface curved corresponding to the curvature of the metal backingI9 whereby to fit snugly against it. Pipe connections 22 and 23 providea means for circulating `a heating medium, such as steam, `through theshell. With the parts arranged as described, pressure is applied to theshell 2| through the upper platen II by means of the screw 24. Thebacking I 9, heated by the shell 2l, heats the ing as well ascompressive. The mixture of cellulose acetate and lead carbonatedisclosed in my prior Patent No. 2,219,587 is wellable to withstand thecompressive strains imposed upon it, while the method of joining such aplastic facing to a metallic backing as herein disclosed provides ameans for resisting the shearing strains imposed upon the facing.

I have found that in addition to etching the surface of the metalbacking, as shown in Figs. 3 to 6 inclusive, that the facing may bejoined to the lbacking, especially along the edges of the plate, byproviding holes 28 extending entirely through the backing I9 andcountersunk, as at 29, to provide shoulders on'the underside of thebacking I9. Before placing the backing I9 in the press I0, as shown inFig. 1, thelholes 28 are filled' with a plastic composition compatiblewith that employed in the plastic facing I8, whereby -when the facing issubjected to heat and pressure in the press I0, it fuses with and joinsto the composition in the holes 28, thereby firmly uniting therewith. Aline of such holes 28 may be provided on each Aedge ofthe plate, asshown in Fig.'7. 1

If the etched surface of the backing I9 is to be treated so as to iillthe undercut recesses with plastic before applying the facing, this maybe accomplished by dissolving the plastic composition in a volatilesolvent and by painting the surface of the backing with the dissolvedplastic and allowing the solvent to evaporate. This will completely fillthe undercut recesses so that the material therein will unite with thematerial of plastic facing I8 and causes it to unite with the backing.

As hereinbefore mentioned, if the backing has already been coatedwithplastic so as to fill the recesses in the plate, the faclnguniteswith the plastic already in the recesses. If 'the backing has not beenso treated, the plastic composition of the facing enters the recessesand fills them.

Referring to Figs. 3 and 4 of the drawing, the backing may beprepared`by subjecting its sensitized surface to light through a photoengravers screen'and etching to providel small, undercut, l

reverse dot openings 26. It may also be prepared by subjecting the'sensitized surface of the backing I have found that the strain on acurved plate such as herein disclosed, in operation, is shearto lightthrough'a phot engravers high light,- 5

the plastic facing upon the application of heat and pressure. .I

While I contemplate the use of a cellulosic plastic composition, such asis disclosed in my prior Patent No. 2,219,587, it will be obvious thatany plastic printing plate moldable under heat and pressure may besecured to a metallic backing in accordance with myinvention. Plasticshaving a softening point belowthe temperature of steam at atmosphericpressure lend themselves more economically to handling than do others.Also, where a solvent is to be employed to fill the undercut recesses ofthe backing before applying the facing, a plastic soluble in-a volatilesolvent, such as actone, alcohol, or other suitable solvent ispreferred. Y,

From the foregoing it will be apparent that I have devised an improvedcomposite printing plate embodying a, plasticfacing bearing the printingsurface and a metal backing, and a process of rnakingt the same.

forms, it will be obvious to those vskilled in the art that itis not solimited, but is susceptible of various other changes and modifications,without prises etching the surface of tlie backing to provide undercutrecesses, heating thebacking to a.

temperature above the softening point of the plastic plate, and pressingthe relatively cool plastic, plate against the heated backing to causeit to engage therewith, and maintaining the outer surface of the plasticsheet cool.

" 2. The method of securing -a thermoplastic printing plate toametallicbackingwhich comdeparting from the spirit thereof, and I.desire, therefore, that'only such limitations shall be ing whichcomprises forming the backing to the required curvature, etching thesame to provide undercut recesses in the convex surface thereof, fillingthe. recesses with a plastic composition compatible with that of theprinting plate, heating the backing to a temperature above the softeningpoint of the plastic facing, pressing the backcof the printing plateagainst the etched surface of the backing, and simultaneously cooling l0the printing face of the plate.

CLARENCE E. lisorrrWriLL.

